Topic Two; Electricity Flashcards
What is current?
Current is the flow of electrical charge.
Electrical current:
Electrical charge will only flow round a complete circuit if there is a potential difference.
Therefore a current can only flow if there’s a source of potential difference.
What is the unit of current?
Ampere (A)
Current in a single closed loop:
In a single closed loop the current has the same value everywhere in the circuit.
What is potential difference? (P.D)
Potential difference (or voltage) is the driving force that pushes the charge round.
What is the unit of PD?
Voltage so: volt (V)
What is resistance:
Resistance is anything that slows the flow down
What is the unit of resistance?
Ohm
The current through a component depends on….
The potential difference across it and the resistance of the components.
The greater the resistance across a component…
The SMALLER the current that flows (for a given potential difference across the component)
The larger the flow of current…
The more charge passes around the circuit
Formula to find the total charge
Q= I x T
Coulombs(c) = current(A) x Time(s)
Formula linking PS and current
Potential difference (v)= current (A) x resistance (ohm)
V=I xR
The resistance of an ohmic conductor:
A. Doesn’t change with the current
B. Does change with the current
+ points about ohmic conductors
For some components, as the current through the is changed, the resistance of the component changes aswell
The resistance of ohmic conductors (e.g a wire or a resistor) doesn’t change with the current.
- At constant temperature the current flowing through an ohmic conductor is directly proportional to the potential difference across it. (R is constant in V= IR)
Diodes and filament lamps
The resistance of some resistors and components DOES change. E.g in a diode or filament lamp.
When an electrical charge flows through a filament lamp, it transfers some energy to the thermal store of the filament, which is designed to heat up. Resistance increases with temperature, so as the current i creases, the filament heats up more and the resistance increases.
For diodes, the resistance depends on the direction of the current. They will only let current flow in one direction but they have a very high resistance if reversed
Ohmic conductor graph p.g 26
It’s a straight line at x=y
The current through an ohmic conductor (at a constant temp) is directly proportional to potential difference so you get a straight line.
If the temperature of any resistor goes up …
The resistance goes up except from a thermistor.
A thermistors resistance drops in hot conditions.
Filament lamp graph p.g 26
It’s an s shape
As the current increases the temp of the filament increases, so the resistance increases. This means less current can flow per unit pd, so the graph gets shallower- hence the curve.
Diode graph p.g 26
Starts from 0.7 (don’t need to know)
Current will only flow through a diode in one direction, as shown. The diode has a very high resistance in the reverse direction.
What is an LDR?
What does it do?
LDR= light dependant resistor
It’s a resistor that is dependant on the intensity of light
In bright light, the resistance falls
In darkness, the resistance is highest
Applications: used for automatic night lights
Outdoor lighting and burglar detectors
What is a Sensing circuit
Sensing circuits are used to turn on or increase the power to components depending on the conditions that they’re in.
Sensing circuit used to control a fan:
The fixed resistor and the fam will always have the same potential difference across them because they’re connected in parallel.
The pd of the power supply is shared out between the thermistor and the loop made up of the fixed resistor and the fan according to their resistances - the bigger a components resistance, the more of the pd it takes.
So as the room gets hotter, the resistance of the thermistor decreases and takes a smaller share of the pd from the power supply. (This is because In hot conditions, the thermistors resistance drops. )So the pd across the fixed resistor and the fan rises, making the fan go faster.
A sensing circuit for a variable resistor
ASK MR SAEED IF THERES A MISTAKE OM THE GREEN BIT END OF SECOND SENTENCE.
If you connect a bulb in parallel to an LDR the pd across both the LDR and the bulb will be high When it’s dark and the LDRS resistance is high
The greater the pd across a component the more energy it gets. So a bulb connected across an LDR would get brighter as the room got darker.
What are the two types of circuits?
Series
Parallel
